Pulling nozzle for oriented pulling of semiconductor crystals from a melt



July 16. 1968 .1. RUPPRECHT ET AL 3,393,054 PULLING NOZZLE FOR ORIENTED PULLING OF SEMICONDUCTOR CRYSTALS FROM A MELT Filed Sept 10, 1965 United States Patent 7 Claims. ci. 23-273 ABSTRACT OF THE DISCLOSURE A pulling nozzle for pulling a crystal with axially oriented crystallites from a melt in a crucible which has a lower opening comprises a first part of quartz or boron nitride positioned next adacent the lower opening of the crucible. A second part of boron carbide, molybdenum silicide or graphite is coaxially positioned with the second part contacting the first part transverse to the axis and positioned farther from the lower opening of the crucible than the first part. The material of the first part is difierent from that of the second part and has a lower heat conductivity than that of the second part. An axial bore exends through the first and second parts and conducts the melt.

The present invention relates to a pulling nozzle for pulling crystals from a melt. More particularly, the invention relates to a pulling nozzle for oriented pulling of semiconductor crystals from a melt.

Thermoelements of bismuth telluride alloys require that the semiconductor crystals have an axial orientation of the crystallites which is as complete as possible. Since the semiconductor crystals are obtained by pulling from a melt, it is diflicult to provide the desired axial orientation of the crystallites.

The principal object of the present invention is to provide a new and improved pulling nozzle for oriented pulling of crystals from a melt.

An object of the present invention is to provide a new and improved pulling nozzle for pulling of semiconductor crystals from a melt with axially oriented crystallites.

Another object of the present invention is to provide a new and improved pulling nozzle for pulling of semiconductor crystals from a melt with axially oriented crystallites and good dissipation of heat from the semiconductor crystals.

Another object of the present invention is to provide a new and improved pulling nozzle for pulling of semiconductor crystals from a melt with axially oriented crystallites and high pulling velocity.

In accordance with the present invention, a pulling nozzle for pulling crystal with axially oriented crystallites from a semiconductor melt positioned above the pulling nozzle in proximity therewith has an axis and comprises an upper part of a first material positioned in close proximity with the melt and a lower part of a second material coaxially positioned with the lower part contacting the upper part transverse to the axis and positioned farther from the melt than the upper part. The upper part has a lower heat conductivity than the lower part.

In order that the present invention may be readily carried into eifect, it will now be described with reference to the accompanying drawing wherein:

FIG. 1 is a view, partly in section, of an embodiment of the pulling nozzle of the present invention in use with apparatus for producing a crystal from a melt; and

FIG. 2 is a view, partly in section, of another embodiment of the pulling nozzle of the present invention.

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In FIG. 1, a heating or melting crucible 11 is positioned in a furnace comprising a ceramic receptacle 12 having a heating coil therein in the form of a molybdenum heating wire 13. A lining 14 is provided inside the heating crucible 11 to direct a melt 15 continuously to the inlet opening 16 of a pulling nozzle 17. Each of the melting crucible 11 and the crucible lining 14 may comprise the same material such as, for example, bornitrite or graphite. The melt 15 is a bismuth telluride alloy, which is a semiconductor.

The pulling nozzle 17 pulls a semiconductor crystal 18 from the melt 15. Any suitable movement actuating and control system may be utilized to control the movement of the pulling nozzle 17. A movement actuating and control system is not shown in the figures in order to maintain the clarity of illustration. The semiconductor crystal 18 passes through a heat exchanger 19.

In accordance with the present invention, the pulling nozzle 17 comprises two parts 21 and 22 coaxially positioned, and contacting each other transverse to the axis of said pulling nozzle, the upper part 21 having an upper surface in contact with the bottom of the lining 14 of the crucible 11 and a lower surface in contact with the upper surface of the lower part 22. The lower part 22 has an upper surface in contact with the lower surface of the upper part 21 and a lower surface in the heat exchanger The upper part 21 of the pulling nozzle comprises a material having a lower heat conductivity characteristic than the lower part 22 of said pulling nozzle. The material of the upper part 21 of the pulling nozzle 17 may comprise boron nitride or quartz. Boron nitride has a heat conductivity characteristic of 0.03 to 0.07 calorie per cm./sec./degree. The material of the lower part 22 of the pulling nozzle 17 may comprise boron carbide, molybdenum silicide or graphite. Boron carbide has a heat conductivity characteristic of 0.2 to 0.7 calorie per cm./sec./ degree.

The different heat conductivities of the upper and lower parts 21 and 22 of the pulling nozzle 17 provide a particularly large temperature gradient in the freezing or solidification zone and result in improved axial orientation of the crystallites of the semiconductor crystal 18. The temperatrue gradient may be even further increased if the upper part 21 of the pulling nozzle 17 is additionally heated and the lower part 22 of said pulling nozzle is additionally cooled. The magnitude of the temperature gradient may be controlled by the heating apparatus utilized.

Since the lower part 22 of the pulling nozzle 17 may comprise material having a considerably higher heat conductivity than that of the usual pulling nozzle material, said pulling nozzle of the present invention provides good dissipation of heat from the semiconductor crystal 18 which is better than that of said usual pulling nozzle. Furthermore, the pulling velocity of the pulling nozzle 17 of the present invention is high and is higher than that of the usual pulling nozzle.

The contacting surfaces of the upper and lower parts 21 and 22 of the pulling nozzle 17 may be stepped, as shown in FIG. 1, so that each forms a shoulder or shoulders upon which the other rests. In the embodiment of FIG. 1, the central part of the upper portion of the lower part 22 of the pulling nozzle 17 extends into the central part of the lower portion of the upper part 21 of said pulling nozzle. This influences the direction of the temperature gradient in the freezing or solidification zone. A convex, viewed from the melt, solidification zone 23 is produced by the aforedescribed configuration of the upper and lower parts 21 and 22 of the pulling nozzle 17. The

3 convex solidification zone 23 results in good axially oriented crystallites.

The desirable good orientation of the crystallites of the semiconductor crystal may be enhanced by an additional heating device 24, which may comprise for example, an induction heating coil, in the area of the solidification zone 23. The magnitude of the temperature gradient may be increased, as previously mentioned, by rapid heat dissipation from or cooling of the semiconductor crystal 18 and the lower part 22 of the pulling nozzle 17. This is accomplished by the heat exchanger 19.

In the embodiment of FIG. 2, the contacting surfaces of the upper and lower parts 21 and 22' of the pulling nozzle 17 are substantially conical in configuration, one of said parts extending into the other. In the embodiment of FIG. 2, the upper portion of the lower part 22 of the pulling nozzle 17' extends into the lower portion of the upper part of said pulling nozzle and is substantially conical in configuration. A convex, substantially rotaryelliptical, viewed from the melt, solidification zone 23' is produced by the aforedescribed configuration of the upper and lower parts 21 and 22 of the pulling nozzle 17 of.

FIG. 2. The configuration of the solidification zone 23 of FIG. 2 is especially conducive to the production of good axially oriented crystallites.

In an actual drawing operation, very well axially oriented semiconductor crystals were obtained from a bismuth telluride melt in a graphite crucible under an argon atmosphere by the pulling nozzle 17 of FIG. 2 having a pulling channel diameter of mm. and an outer diameter of 20 mnr, at a pulling velocity of from 1 to mm. per minute.

While the invention has been described by means of a specific example and in specific embodiment, we do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. Apparatus for pulling a crystal with axially oriented crystallites from a melt, said apparatus comprising a crucible containing a melt and having a lower opening; heating means in operative proximity with said crucible; and a pulling nozzle adjacent the lower opening of said crucible, said pulling nozzle having an axis and comprising a first part of a first material selected from the group consisting of quartz and boron nitride positioned next adjacent the lower opening of said crucible, a second part of a second material selected from the group consisting of boron carbide, molybdenum silicide; and graphite coaxially positioned adjacent said first part contacting said first part transverse to said axis and positioned farther from the lower opening of said crucible than said first part, the material of said first part having a lower heat conductivity than the material of said second part, and an axial bore extending through said first and second parts and conducting the melt.

2. Apparatus as claimed in claim 1, wherein each of 4 said first and second parts has a portion adjacent the other, each portion having a central part, and the central part of the adjacent portion of said second part extends into the central part of the adjacent portion of said first part.

3. Apparatus as claimed in claim 1, wherein each of said first and second parts has a portion adjacent the other, each portion having a central part and the central part of the adjacent portion of said second part extends into the central part of the adjacent portion of said first part in stepped configuration, each of said first and second parts forming a shoulder upon which the other rests.

4. Apparatus as claimed in claim 1, wherein each of said first and second parts has a portion adjacent the other, each portion having a central part, and the central part of the adjacent portion of said second part extends into the central part of the adjacent portion of said first part in substantially conical configuration.

5. Apparatus for pulling a crystal with axially oriented crystallites from a semiconductor melt, said apparatus comprising a crucible containing a melt and having a lower opening; heating means in operative proximity with said crucible; and a pulling nozzle adjacent the lower opening of said crucible, said pulling nozzle having an axis and comprising an upper part of a first material selected from the group consisting of quartz and boron nitride positioned next adjacent the lower opening of said crucible, and a lower part of a second material selected from the group consisting of boron carbide, molybdenum silicide, and graphite coaxially positioned with said lower part contacting said upper part transverse to said axis and positioned farther from the lower opening of said crucible than said upper part, the material of said upper part having a lower heat conductivity than the material of said lower part, and an axial bore extending through said first and second parts and conducting the melt.

6. Apparatus as claimed in claim 5, wherein each of the upper and lower parts has a portion adjacent the other, each portion having a central part, and the central part of the adjacent portion of said lower part extends into the central part of the adjacent portion of said upper part.

7. Apparatus as claimed in claim 5, wherein each of the upper and lower parts has a portion adjacent the other, each portion having a central part, and the central part of the adjacent portion of said lower part extends into the central part of the adjacent portion of said upper part in substantially conical configuration.

References Cited UNITED STATES PATENTS 10/1964 Jones 23273 5/1966 Bennett 23-273 

